Axial rotary piston engines with internal axes and two rotors or runners rotating in the same direction, in which inner and outer rotors with profiled flanks formed according to the rules of gear tooth system similarly to an outer and inner gear tooth system mesh with one another, are divided into three types according to the criteria introduced in this field by Wankel's publication "Classification of rotary piston engines" (1963, Deutsche Verlag-Anstalt GmbH, Fachverlag Stuttgart). According to this, in a socalled "KA-engine" the curve generation points lie on the outer rotor and in a so-called "KI-engine" they lie on the inner rotor, whilst in an engine with a third kind of engagement the curve generation comes about through surfaces that sometimes touch and make rolling contact in the manner of a gear tooth system.
The choice of the appropriate type of engine, i.e. the profiling or curve generation of the interengaging tooth flanks, depends essentially on the performance and the volume throughput of the rotary piston engine; harmful flow constrictions and dead spaces, i.e. the volume not displaced by the pistons of the inner rotor penetrating into the working areas, should be avoided or reduced as much as possible. These requirements often contradict one another, e.g. in order to obtain greater rigidity of the rotary piston engine it is often not possible to avoid cutting away the ends of the outer rotor parts that taper towards the periphery even though at the same time this results in larger dead spaces having to be accepted.
A rotary piston engine is known from the German Offenlegungsschrift No. 24 56 252 in which the inner rotor has bulbs acting as pistons which are adapted to the shape of recesses in the outer rotor that form working areas and whose generating curve is shaped as a trochoid. The bulbous pistons of the inner rotor are kinematically generated by sealing strips arranged in grooves in crests between the recesses of the outer rotor; i.e. it is a so-called "KA-engine" and the contour of the inner rotor is determined by the radially-inward projecting sealing strips, as a function of the movement of the two rotors relative to one another and relative to the engine housing. As a result of the working areas being sealed by means of the sealing strips, a non-contact gap seal is achieved in this known rotary piston engine, i.e. except at the sealing strips there is no contact between the profiled flanks of the inner and outer rotors. This has the particular advantage that in the manufacture of the two rotors no exact manufacturing tolerances have to be kept to; against this however there is the disadvantages, arising from the trochoidal generating curve of the bulbous curves of the inner rotor, generated by the sealing strips, of larger harmful dead spaces in the working areas. Especially when semicircular sealing strips engage at their radial ends in the root between two pistons, the unavoidable dead space is still further increased. When the engine is used as a compressor this can lead to already compressed gas being returned to the intake side of the engine.
From the German Offenlegungsschrift No. 34 32 915 an internal axis rotary piston engine is known which represents a combination of the types of engagement of the three known engines; namely in each operational phase there is a direct connection between the contour of the inner and outer rotor such that in every rotational phase the rotors exert an effect which determines (i.e. generates) the contour of the other rotor and that at the same time gear tooth-like engagement of the inner and outer rotors is simultaneously realised. So that both rotors mutually generate or determine their form by their relative movement, corner regions of the inner rotor move along the inner side surfaces of the outer rotor and inner corner regions of the outer rotor move along the outer peripheral surfaces of the inner rotor. Through this mutual influence the volume not displaced by the pistons of the inner rotor that penetrate into the working areas is reduced, i.e. the harmful dead spaces are reduced down to a negligible size by the optimization of the rotor contours. In this way a compromise is reached in which on the one hand dead spaces are reduced to a negligible size without on the other hand causing unduly high or unwanted restricted flows. Realising this aim, however, requires, when manufacturing this known rotary piston engine, highly expensive manufacturing techniques and high precision of the contours of the profiled flanks of the inner and outer rotors which constantly mesh with one another.